x/net: Failure to send IPv4 UDP packet on ipv6 socket with ControlMessage

[Lalufu]

Go version

go version go1.23.2 (Red Hat 1.23.2-1.el9) linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='~/.cache/go-build'
GOENV='~/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='~/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='~/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/golang'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.2 (Red Hat 1.23.2-1.el9)'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='~/.config/go/telemetry'
GCCGO='/bin/gccgo'
GOAMD64='v2'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='~/temp/test/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2023383051=/tmp/go-build -gno-record-gcc-switches'

What did you do?

The following program successfully receives and sends traffic on both IPv4 and IPv6:

package main

import (
        "fmt"
        "golang.org/x/net/ipv6"
        "net"
)

func main() {

        listenSocket, err := net.ListenPacket("udp", ":12345")
        wrapped := ipv6.NewPacketConn(listenSocket)
        wrapped.SetControlMessage(ipv6.FlagDst, false)

        data := make([]byte, 65535)
        n, cm, remote, err := wrapped.ReadFrom(data)
        fmt.Printf("Bytes read %d\n", n)
        fmt.Printf("ReadFrom: %T %+v %+v\n", remote, remote, cm)

        n, err = wrapped.WriteTo(data[:n], cm, remote)
        fmt.Printf("Bytes write %d\n", n)
        if err != nil {
                fmt.Printf("WriteTo: %+v\n", err)
        }

}

Output when receiving traffic on 127.0.0.1:

Bytes read 4
ReadFrom: *net.UDPAddr 127.0.0.1:57398 <nil>
Bytes write 4

Output when receiving traffic on ::1:

Bytes read 4
ReadFrom: *net.UDPAddr [::1]:48275 <nil>
Bytes write 4

When changing false to true in the SetControlMessage() call, the program still works on IPv6, but fails on IPv4.

Output when receiving traffic on 127.0.0.1:

Bytes read 4
ReadFrom: *net.UDPAddr 127.0.0.1:59701 tclass=0x0 hoplim=0 src=<nil> dst=127.0.0.1 ifindex=1 nexthop=<nil> mtu=0
Bytes write 0
WriteTo: write udp [::]:12345->127.0.0.1:59701: sendmsg: invalid argument

Output when receiving traffic on ::1:

Bytes read 4
ReadFrom: *net.UDPAddr [::1]:49933 tclass=0x0 hoplim=0 src=::1 dst=::1 ifindex=1 nexthop=<nil> mtu=0
Bytes write 4

What did you see happen?

Error when sending traffic to an IPv4 address when ControlMessage is used. This is likely because of a confusion in the system call, where the destination address is an IPv4 struct, while the socket options are an IPv6 struct. The fact that the destination address was originally received on an IPv6 socket, and should be passed back into the system as such gets lost.

Example system call capture of a failed IPv4 send:

[pid 48372] recvmsg(3, {msg_name={sa_family=AF_INET6, sin6_port=htons(49970), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_scope_id=0}, msg_namelen=112 => 28, msg_iov=[{iov_base="foo\n", iov_len=65535}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32, cmsg_data="\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x7f\x00\x00\x01\x01\x00\x00\x00"}], msg_controllen=40, msg_flags=0}, 0) = 4
[pid 48372] getsockopt(3, SOL_SOCKET, SO_PROTOCOL, [17], [4]) = 0
[pid 48372] sendmsg(3, {msg_name={sa_family=AF_INET, sin_port=htons(49970), sin_addr=inet_addr("127.0.0.1")}, msg_namelen=16, msg_iov=[{iov_base="foo\n", iov_len=4}], msg_iovlen=1, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32, cmsg_data="\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00"}], msg_controllen=40, msg_flags=0}, 0) = -1 EINVAL (Invalid argument)

What did you expect to see?

Traffic being able to be sent with both IPv4 and IPv6, as is possible without using ControlMessage

[gabyhelp]

Related Issues

• net: WriteMsgUDPAddrPort should accept IPv4 destination addresses on IPv6 UDP sockets #52264 (closed)
• x/net/ipv6: ControlMessage considers HopLimit==0 as missing #41820
• x/net/{ipv4,ipv6}: MSG_DONTWAIT flag ignored #46891 (closed)
• x/net/{ipv4,ipv6}: SetControlMessage not implemented on windows #27966 (closed)
• x/net/route: TestFetchAndParseRIB fails on IPv4-only or IPv6-only kernel #19298 (closed)
• x/net: invalid address when reading socket error queue with SO_TIMESTAMPING #47926
• net.Dial fails with an ip6 addres - connect: invalid argument #43950 (closed)
• proposal: net: UDPAddr/TCPAddr AddrPort should map 4in6 addresses to IPv4 netip.Addrs #53607
• net: sending UDP datagrams back and forth fails with "sendmsg: operation not permitted" #63322

Related Code Changes

• net: fix WriteMsgUDPAddrPort addr handling

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[mknyszek]

CC @neild